Hello everybody...
I am new in the world of game programming and just learn XNA 4.0.
I want to make a racing game like NFSU 2, but more simple.
I am confused on some points

how to make the track? using model (FBX / X) or made by using Catmull? for the collision, using StaticMesh or triangle?
my track must have left boundary and right boundary, so the car NEVER leave the track

As far as the physics is concerned, the track's collision representation will likely end up being represented by meshes of triangles regardless of where they come from (generated from a curve or exported from a modelling program). The StaticMesh is indeed a good choice; it has a specialized hierarchy for lots of triangles. StaticMesh will be much faster than just individually adding thousands of independent triangles.